1. Field of Invention
The present invention relates to a method for preparing a copolyester-ether film. More particularly, the present invention relates to a method for preparing a thermoplastic copolyester-ether film from recycled polyester.
2. Description of Related Art
The basic molecular structure of thermoplastic copolyester-ether (or thermoplastic polyester elastomer, TPEE) includes a hard segment able to form crystalline regions, and a soft segment able to maintain the elasticity and flexibility of the copolyester-ether. The crystalline regions are formed by the orderly arranged hard segment. By changing the species and controlling the amount of the hard and soft segments, TPEE exhibits various physical properties. Therefore, there are several key factors in determining the physical properties of TPEE such as the molecular composition and structure of the hard and soft segments, the amount and molecular weight of monomers, and the ratios of the hard and soft segments in TPEE.
In general, polyesters capable of crystallization are candidates as hard segment, such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT). Long-chain polyalkylene glycol is usually used as soft segment, such as polyethylene glycol (PEG) and polytetramethylene glycol (PTMEG). Currently, industrially produced TPEE mainly uses PBT as the hard segment and PTMEG as the soft segment, which is mostly applied in sports equipment, car interior materials, etc.
Concerning the characteristics of material, TPEE using PBT as the hard segment has faster crystallization rate, and thus provides better dimensional stability and shortens the process cycle in the applications of injection molding for manufacturing machine parts, such as automotive tubing and sheath of wire and cable. TPEE using PET as the hard segment is more heat resistant than TPEE using PBT as the hard segment because PET has a higher melting point. Though the crystallization rate is slower than PBT, it is easier for TPEE using PET as the hard segment to produce semi-crystalline domain in that PET includes short molecular chains of ethylene molecular segment. Therefore, the crystallization of TPEE is adjustable by the ethylene to control the crystallization ratio of the hard segment. Given the above, TPEE adjusts the rigidity of crystalline of the hard segment, the flexibility of the soft segment, and the freedom of movement of the molecular chain segment to achieve a balance between the ratio of the hard and soft segment, so as to provide the material with great elasticity and strength at the same time.
The conventional preparation of a copolyester-ether uses dimethyl terephthalate (DMT) and diol, such as butanediol (BG) to perform transesterification in the presence of a transesterification catalyst. However, there is difficulty in the conventional method that byproducts are also easily produced during transesterification, e.g. methanol and tetrahydrofuran (THF) of low-molecular-weight alcohols. The byproducts may cause side reactions or reverse reactions during transesterification, and lead to problems like uneven distribution of molecular weight of product and poor mechanical properties.
Taiwan Patent No. 1232226 discloses a method for preparing a copolyether ester, which applies a method similar with the above method in the art with difference of using a mixed catalyst of titanium(IV)-n-butoxide and magnesium acetate to replace the single titanium catalyst to reduce the reaction time of polycondensation. No disclosure of inhibiting side reactions in the art. U.S. Pat. Nos. 7,795,320 and 8,110,609 disclose a polytrimethylene ether ester block copolymer and a method for preparing the same respectively, and both use PET scrap as reactant. The techniques disclosed is to alcoholysis the PET scrap to form dimethyl terephthalate (DMT), and additional dimethyl dimethyl isophthalate (DMI) having similar molecular structure but lower melting point has to be added to improve the degree of transesterification. Afterwards, a polycondensation reaction is performed by reacting with polytetramethylene glycol (PTMEG) to form the polytrimethylene ether ester block copolymer. However, PTMEG will dissociate to tetrahydrofuran (THF) at high temperature, and THF may be harmful to humans and the environment. Therefore, the polytrimethylene ether ester block copolymer is not suitable for products, such as fabric film and foil for furniture supplies, in direct contact with human body. Moreover, the main chain of the molecular structure of PTMEG is a linear carbon chain with four carbon atoms, and the linear carbon chain is prone to have preferred crystallization orientation, and thus is suitable for injection molded or extrusion molded. The injection molding needs material having high crystallization speed to reduce the cycle time, and the extrusion molded also needs material to have characteristic of high crystallization speed. Therefore, the material characteristic of being prone to have preferred crystallization orientation is more suitable for injection molding or extrusion molding. However, the material which crystallized in preferred orientation too fast is not suitable for application of making a film by extension molding because during the extension molding, too-fast crystallization may cause problems like broken film or film with uneven thickness. In addition, it is difficult to make a film that fits the required thickness in fabric, in which the waterproof breathable film used in fabric usually has a thickness of about 20 μm. Moreover, U.S. Pat. No. 6,380,290 discloses an engineering plastics level thermoplastic segmented polyetherester copolymer having a melting point above 200° C. Aromatic amine thermo oxidative stabilizer is added to enhance the anti-oxidative properties to improve the weather-ability of the material. However, the flexibility of the polyetherester copolymer cannot be improved. U.S. Pat. No. 7,799,838 discloses a modified copolyetherester formed by physical blending. However, the flexibility of the copolyetherester cannot be improved either.
Accordingly, there is a need for a method for preparing a copolyester-ether film that has characteristics of both PBT and PET as the main structure of the hard segment to provide an adequate crystallization speed and the convenience of making a film by extension molding, so as to provide the film material with greatly improved flexibility. The material of the copolyester-ether film prepared by the method of the present invention applied in fabric clothing can be soft and comfortable to wear. Moreover, the method for preparing a copolyester-ether film of the present invention does not dissociate to harmful substances during the preparing process, which is environmentally friendly, and can be safely used in clothing that needs long-term contact with human body.